Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial applications because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for molding articles for which impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
It is known that polycarbonates exhibit high notched Izod (ASTM D-256) impact values. These values, about 16 ft.lb/in., associated with failure in a ductile mode, are characteristic of test specimens thinner than about 1/8 inch. Thicker specimens, i.e., 1/4 inch, fail in a brittle fashion at about 2.5 ft.lb/in.
Impact strength dependence on thickness gives rise to "critical thickness", a material parameter defined as that thickness at which a discontinuity occurs in the graphical representation of impact strength vs. specimen thickness. For example, polycarbonate based on bisphenol A having a melt flow rate of 3 to 6 grams/10 minutes at 300.degree. C. (ASTM D-1238) exhibits a discontinuity at about 5.71 mm (225 mils). Articles of this material thinner than 225 mils will show ductile failure upon impact while thicker ones will fail in a brittle fashion.
This dependence of impact performance on thickness represents a drawback because it tends to limit the utility of polycarbonates by imposing restrictions on the design of molded articles.
Among the means disclosed in the art to improve the impact performance of polycarbonate resins, acrylic rubber interpolymer composites were disclosed in U.S. Pat. No. 4,299,928 and butyl acrylate rubbers were disclosed in U.S. Pat. No. 3,742,088.
Polycarbonates characterized by their alkylphenyl end groups are noted for their levels of resistance to hydrolysis and critical thickness which are improved over those of conventionally terminated polycarbonate resins. Alkylphenyl terminated polycarbonate resins are more particularly described in U.S. Pat. No. 4,269,964, incorporated herein by reference.